In a standard boiler mechanism, burnt combustion gases are exhausted through a separate output from the steam output. In a direct fired boiler, such burnt combustion gases are exhausted through the same output as the steam. Various direct fired boiler mechanisms are known in the prior art for producing an output of steam. The prior art lacks, however, a pressure controlled variable output direct fired steam boiler. Such a boiler would ideally monitor its own output and control its own consumption of fuel in response thereto. Such a mechanism should also vary the supply of water delivered to the boiler for conversion to steam in response to the necessary amount of water required to achieve a particular desired output. Finally, such a mechanism should provide an adequate supply of combustion air to the combustion chamber.